ADA4941-1 outputs in-phase... cause?

Hello,

while measuring  the specs of a measurement prtotype, where at the end a
ADA4941 drives an AD7691, I noticed that the outpunts of the differential
amplifier are in phase. I could not detect shorts and measuramnts were taken
with and without load.

My question is now: Does this indicate a broken IC or is it possible to get
OUT_+ = OUT_- from the SE to DM Amplifier, while builidng a driving stage
according to p. 22 of the datasheet?

Regards,

B.

  • 0
    •  Analog Employees 
    on Jan 5, 2017 10:20 PM

    Hi Wojtek,

    No, the +OUT and -OUT should be 180degrees out of phase with each other. If that's the case, when the outputs are in phase at the ADC inputs, the ADC will just convert the Output Balance Error of the outputs of ADA4941 and not the differential of the two. So it's not normal that +OUT and -OUT are in phase, obviously something's wrong. Could you share your schematic as well as the test setup you are working with please? Thank you!

    Regards,

    Jino

  • Hi Jino,

    I already thought that in-phase outputs are wrong. Therefore, I switched the IC for a new one. Unfortunately, nothing has changed regarding the problem.

    I really hope you will be able to help me, as I used the circuitry from the datasheet and checked all supply voltages three times, and I still cannot find any errors on my side.

    The project is made in KiCad. The setup is photographed. On the scope, magenta is the input, before the last filter circuit, the other two curves are the outputs. What I noticed, if I pull either output via resistor to ground, both outputs go low together. That seems strange to me.

    Looking forward to any help. Thank you!

    Regards,

    Wojtek

    attachments.zip
  • 0
    •  Analog Employees 
    on Jan 12, 2017 7:42 PM

    Hi Wojtek,

    Your circuit using ADA4941-1 should work fine. I tried it thru simulations and it is not in-phase. Could you make sure that the contacts are clean? I mean no short-circuits. And could I have your pcb layout please? Just to make sure.

    Regards.

  • Hi JinoL,

    i posted the whole KiCad project folder as a zip in my last post.

    I know everything should work. That's why I don't know what to change or try anymore. And I probed every voltage, in full and in partial circuitry. That means, I opened any tracks in the circuit I could to see what will change, but unfortunately nothing helped.

    Also, in my last post, there are pictures of the setup together with an oscilloscope in the back, and the outputs are in phase. If I measure differentially between output line I only see noise.

    I hope you will have an idea, after looking over the layout, as I'm running out of those.

    regards,

    W.